This invention relates to an improved process for the production of methyl acetate from dimethyl ether, and more generally to the production of alkyl esters of aliphatic carboxylic acids, by the carbonylation of alkyl ethers. In another aspect this invention relates to the production of lower aliphatic carboxylic acids by first producing an alkyl ester from a lower alkyl ether, followed by hydrolysis of the ester to the acid. An example of this is the production of acetic acid by carbonylation of dimethyl ether, to form methyl acetate, followed by hydrolysis of the ester to produce acetic acid.
The most widely used industrial process for production of acetic acid is the carbonylation of methanol, which is described generally in British patents 1,185,453 and 1,277,242 and U.S. Pat. No. 3,689,533, for instance. In that type of process, methanol is reacted with carbon monoxide or a carbon monoxide-containing gas in the presence of a rhodium- or iridium-containing catalyst, in the additional presence of a halogen (usually iodine)-containing promoter. Though widely used, nonetheless these processes require the use of expensive corrosion-resistant alloys due to the presence of iodide and result in production of low levels of iodine-containing byproducts that are difficult to remove from the acetic acid by conventional distillation. Some non-halide based catalyst systems have been investigated for this reaction, but none have been commercialized, primarily due to issues with catalyst lifetime and selectivity.
Methyl acetate is an important compound used industrially in petrochemical processes, particularly as a feed for the production of acetic anhydride and/or acetic acid. Methyl acetate can also be used for the production of ethylidene diacetate, a precursor to vinyl acetate and polyvinyl acetate. Dimethyl ether may be readily produced from synthesis gas, and the cost of its production can be lower than that of methanol.
A number of patents describe processes in which methanol or a mixture of methanol and dimethyl ether is carbonylated in the presence of a catalyst. Typically the products are a mixture of acetic acid and methyl acetate, sometimes also including acetic anhydride. In those patents it is disclosed that one of the reactions that may occur is the carbonylation of dimethyl ether to form methyl acetate. Typically, however, dimethyl ether is not used as the sole or even as the primary component of the feed, but as a minor component in methanol streams.
For example, German OLS 3,606,169 of BASF AG discloses carbonylation of a mixture of methanol, methyl acetate and/or dimethyl ether to produce a product containing acetic acid, methyl acetate and/or dimethyl ether in the presence of a cobalt-containing zeolite catalyst. The preferred zeolites are those of the 10-ring pentasil type with pore sizes intermediate between those of 8-ring zeolite A and those of 12-ring zeolites X and Y.
Jones et al., U.S. Pat. No. 6,130,355, disclose a process for carbonylation of methanol and/or dimethyl ether to produce acetic acid using a catalyst composed of at least one Group VIII noble metal, a halogenated compound as cocatalyst, and an iodide salt as catalyst stabilizer. Other patents disclosing processes for production of acetic acid and/or methyl acetate, in which dimethyl ether may be present in the feed in a mixture with methanol include U.S. Pat. Nos. 6,353,132 and 6,355,837 and U.S. published application 2003/005495.1, all of Zoeller et al. U.S. Pat. Nos. 5,189,203, 5,286,900 (both of Hansen et al.) and 5,728,871 (Joensen et al.) disclose processes in which syngas is first used to produce methanol, which is then combined with dimethyl ether, and the mixture carbonylated to produce acetic acid as a major product.
Several other references investigated carbonylation of dimethyl ether as the primary or sole component of a feed using various catalysts. For example, Jones et al. (U.S. Pat. No. 5,763,654) disclose such a process in which the catalyst is a Group VIII noble metal catalyst, with a halide-containing cocatalyst and methyl iodide as a promoter. Water was present in the reactor, though according to the disclosure of this patent it was used at lower concentrations than typical in the prior art. The major product was acetic acid.
Wegman (U.S. Pat. No. 5,218,140) experimented primarily with the carbonylation of methanol to produce acetic acid using heteropolyacid catalysts. The patent contains a group of experiments in which the feed was dimethyl ether (Examples 28-33); however, in those experiments the conversion to methyl acetate was relatively low.
Sardesai et al. (Energy Sources 2002, 24:301) also carried out carbonylation of dimethyl ether with a number of heteropoly acid catalysts, which gave results that varied widely in terms of conversion and selectivity to methyl acetate. Bagno et al. (J. Org. Chem. 1990, 55:4284) ran such a reaction with so-called “super-acid” catalysts including BF3 and triflic acid, again with varying results as to selectively to methyl acetate.